The Agricultural Health Study (AHS) is an established, on-going prospective cohort study examining the relationship between agricultural exposures, such as pesticide use, and disease among applicators in Iowa and North Carolina. The AHS in Iowa involves 36,793 licensed pesticide applicators, plus 21,773 spouses who are exposed either directly or indirectly to pesticides and other agricultural exposures. Prostate cancer incidence and mortality are higher among farmers, making this a high-risk cohort. The mechanism of potential exposure among farmers is unclear, so research into this subgroup of Americans is important. Current literature suggests associations with prostate cancer and arsenic, cadmium and low selenium levels but studies have not been consistent. We propose a nested case-control pilot study of incident prostate cancer within the existing AHS, identified through semi-rapid reporting of prostate cancers by the Iowa Cancer Registry (ICR), to examine trace elements found in toenails including arsenic, cadmium and selenium, all long-lived indicators, along with other trace elements that fit into this classification. We will compare these elements in 86 incident prostate cancer cases to 172 controls (frequency matched on age, completion of the diet survey, and a recent PSA test) from within the AHS cohort. We will use neutron activation analysis (NAA) to analyze for the targeted trace elements. We justify examining the other trace elements as a cost efficient, hypothesis generating sub-study. A secondary aim of this pilot study is to examine the validity of arsenic and selenium measured in toenails compared with standard questionnaire measurements of dietary selenium and arsenic pesticides collected prior to diagnosis in this cohort. In residentially stable populations, biomarkers of trace elements may be a good measure of exposure reducing the need for questionnaires with long detailed lists of pesticides and complex food frequency questionnaires (FFQs).